Synchronization of mutually coupled digital PLLs in massive MIMO systems

For next-generation wireless networks, massive multiple-input multiple-output (MIMO) promises significant performance gains compared to today´s wireless communication standards. In this paper, we address the challenge of how to synchronize the carrier signals of all antenna units in a large network. We show that mutually coupled digital phase-locked loops (DPLLs) can enable in-phase synchronous clocking in largescale systems with transmission delay. Using a phase model of coupled DPLLs including signal filtering and signal transmission delays, we show how the collective frequency and the time scales of synchronization depend on system specifications. To test our theoretical predictions, we designed and carried out experiments, thereby providing a proof-of-principle that mutually delay-coupled DPLLs can provide self-organized synchronous clocking.